She said: “I cannot believe my husband lets me have as much sex as I want with my boyfriend. I am a lucky girl.”

Some couples even use the fetish as part of a game with one husband describing how his wife is allowed to sleep with one other man at a time but couldn’t switch men more than once a month.

“Here is the fun part,” he explained. “She can’t let me catch her or she can’t f*** that guy for three months.”

The cause behind the rise of this sexual fetish isn’t clear but some psychologists have suggested it could be due to repressed male bisexuality or men being proud of their wives’ unrestrained sex lives.

One site sees user submitting pictures of their own wives for others to rate

One way in which the fetish could start is by participating in online communities, such as one found on the website Reddit, where users post pictures of their wives and ask other men to comment on them.

Dr David Ley, who wrote the book ‘Insatiable Wives: Women Who Stray and the Men Who Love Them’, said it may be due to the simple act of doing something so taboo.

He told Psychology Today: “It’s essential to grasp that what might be humiliating about imagining one’s wife having sex with another male is, in its idealized formulation, transformed into something not humiliating at all but engrossingly erotic.”

He added: “When an otherwise well-controlled heterosexual male dares to visually create his wife’s violating her marital vows, and possibly his even encouraging her to do so, he’s playing a vital role in what we might call a ‘double transgression’ of society’s norms.

“Voluntarily fantasizing himself as a cuckold, yet fully in charge of his cuckoldry, his ‘forbidden fantasies’ may be particularly gratifying.”

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